1. Field of the Invention
The invention relates to a light source device, and more particularly to a light source device capable of repressing the light surge by utilizing obscuring units to obscure portions of the light source of the bent lamp.
2. Description of the Related Art
FIG. 1 shows a partial architecture diagram of a conventional flatbed scanner. Referring to FIG. 1, the flatbed scanner includes a carriage 14 and a scan platen 12. A to-be-scanned document 11 is placed on the scan platen 12. The carriage 14 includes a light source device (lamp) 13, mirrors 15, a lens 16, and an image sensor 17. The image sensor 17 may be a charge coupled device (CCD). The light source device 13 includes a lamp 131 and a lamp holder 132. The carriage 14 has an image signal inlet 18 at a position close to the light source device 13, such that the scattered light reflected from the to-be-scanned document 11 may enter the carriage 14 via the image signal inlet 18 while the light of the lamp 131 illuminates the to-be-scanned document 11. The light entering the carriage 14 is reflected by the mirrors 15 and then projected onto the image sensor 17 through the lens 16.
FIG. 2 shows a schematic illustration of a conventional linear lamp applied in a flatbed scanner. As shown in FIG. 2, the lamp 21 has a length of L0 and provides the effective scan width of X, wherein L0 is greater than X. Symbol 18 denotes the image signal inlet 18 of the flatbed scanner. Because the brightness at two ends of the typical lamp (e.g., cold cathode fluorescent lamp) is weaker, the effective scan width X is smaller than the length L0, and some positions (e.g., areas a and b) at the two ends of the lamp 21 cannot provide the sufficient brightness.
In order to compensate for the weaker brightness at the two ends of the typical linear lamp 21 of FIG. 2, a bent lamp of FIG. 3A is proposed. As shown in FIG. 3A, the lamp 31 has two U-shape structures at two ends, and each of the U-shape structures has a first compensation section C1 and a second compensation section C2 for respectively compensating for the weaker brightness at two ends of the typical lamp 21. Hence, it is noted from comparing FIG. 2 to FIG. 3A that the length L1 of the lamp 31 of FIG. 3A may be smaller than the length L0 of the lamp 21 of FIG. 2. FIG. 3B shows a light path from the lamp to the image sensor. The symbol θ is the lens half angle and the symbol α is the light angle from the lamp 32 to the terminal of the image signal inlet 18.
FIG. 4 shows the brightness variations detected by the image sensor from the lamps of FIGS. 2 and 3, wherein the curve 41 denotes the brightness variation of the lamp 21 of FIG. 2, and the curve 42 denotes the brightness variation of the lamp 31 of FIG. 3A. It can be understood from FIG. 4 that better brightness may be provided at two ends of the image signal inlet 18 when the bent lamp of FIG. 3A is utilized. However, if the lamp 31 provides the brightness representation of the curve 42 of FIG. 4, it should be noted that the distance between the point A and the point C at two ends of the lamp 31 of FIG. 3A must greater than the effective scan width X. As shown in FIG. 3B, the angle α must be larger than lens half angle θ. That is, the points A and C at two ends of the lamp 31 have to be located outside the points B and D, which define the two ends of the effective scan width.
FIG. 5 shows a schematic illustration showing another conventional bent lamp applied in a flatbed scanner. Each of the lamps of FIGS. 5 and 3 has the U-shaped structures at two ends thereof. But the angle α in FIG. 5 system is smaller then lens half angle θ. That's, the difference between FIGS. 5 and 3 is that the distance from points A to C at two ends of the lamp 51 of FIG. 5 is smaller, even smaller than the effective scan width X. In the lamp 51 of FIG. 5, the distance from points A to C at two ends of the lamp 51 is smaller than the effective scan width X, so the effective scan width X from points B to D extends to the positions above the lamp. Because the angle α in FIG. 5 system is smaller then lens half angle θ, the brightness received by the image sensor will include the light surge at two ends because of the reason of lamp image formation.
FIG. 6 shows the brightness variations detected by the image sensor from the lamps of FIGS. 2 and 5, wherein the curve 41 denotes the brightness variation of the lamp 21 of FIG. 2, and the curve 43 denotes the brightness variation of the lamp 51 of FIG. 5. The light surge not only tends to cause the over-saturation of the optical signals at two ends of the image sensor, but also makes the color correction computations of the software or firmware unstable and incorrectly, thereby causing abnormally scanned image.
The light surge is generated because the image sensor is used to scan a reflective original, and the light source device is used to provide the light beam. The light beam received by the image sensor is the scattered light but not the reflected light, so the overall utilization efficiency of the light source is originally low, and the difference between the efficiencies of the normally and slantly incident light sources is larger, both of which are the normal physical phenomena. Because the angle α in FIG. 5 system is smaller then lens half angle θ, even the points B and D of the lamp 51 of FIG. 5 extend to the positions above the lamp, a portion of light of the second compensation section C2 is reflected to the image sensor and thus cause the light surge. Hence, the light surge may be avoided without losing the compensating effect if the distance AB and the distance CD exceed some value (depending on the optical system).
Because the distances AB and CD have to exceed some value, the lamp length L1 of FIG. 3A cannot be effectively shortened such that the width of the flatbed scanner also cannot be reduced under the condition that the effective scan width X is fixed. If the lamp 51 of FIG. 5 is utilized to provide the light source, the lamp length can be shortened to L2, but the light surge will be generated again, thereby deteriorating the image quality.